1. Field of the Invention
The present invention relates generally to the field of hybrid collectors and more particularly to a hybrid wet electrostatic collector for collecting sub-micron and nano-particulate material.
2. Description of the Prior Art
There are cases in industrial applications where, before emitting industrial waste gases into the atmosphere such as exhaust gases of boilers in factories or smoke from power generating plants, air purification processing is performed to remove various types of fine particulates contained in these waste gases. These fine particulates include mist and/or dust with powders containing oil, moisture, and/or the like, which can pollute the atmosphere. Direct emission of the industrial waste gases containing the fine particulate into the atmosphere significantly affects the global environment, and hence, it is many times obligatory to perform collection by national or local standards. In addition, in municipal zones, air pollution resulting from automobile exhaust gases is a serious issue causing even ordinary homes to sometimes have and use an air cleaning apparatus. In many sites, such as kitchens of restaurants, there are exhaust cleaning apparatuses for cleaning exhaust gas before it is emitted to the ambient. This can include polluted air and smoke generated during cooking and the like.
A dust collector can be used to collect fine particles contained in an exhaust which can cause air pollution. Dust collectors used to collect fine particles contained in polluted exhaust can be classified into several types based on the collection principle used. They can be classified as filtration, gravitational, inertial, centrifugal, dust precipitation, and wet types, as well as other types. They are normally selected for practical use depending on, for example, the size and type of fine particulate to be collected, and/or the installation conditions of the apparatuses. In particular, of the types described above, the filtration type (using a bag filter or the like), and the dust precipitation type are excellent from the viewpoint of the dust collection capability. These are widely used in various industrial fields. There is also a separate class of the apparatuses incorporating a combination of the above types termed “hybrid” devices. Purely electrostatic precipitators, known in the art, fall into the category of dust precipitation types.
The dust collection principle used by electrostatic precipitators is such that electric charges can be supplied to the fine particulate through corona discharges generated from discharge electrodes, and coulomb forces in other zones of the collector can then be used to electrostatically attract the charged fine particulate onto collector electrodes which are opposed electrodes, whereby the fine particulate is collected. The electrostatic precipitator has significant advantages over other collector types. For example, 1) low pressure loss; 2) a large amount of gas can be processed; and 3) high collection efficiency. For these reasons and others, electrostatic dust precipitators find wide use in such environments as factories, industrial and power generating plants, which emit large amounts of polluted exhaust gases.
Generally, the construction of prior art electrostatic precipitators includes (i) discharge electrodes each formed into a shape having a sharp (small) surface curvature, such as a needle or wire material, for generating corona discharges and supplying electric charges to the fine particulate; (ii) collector electrodes, as opposed electrodes, each formed into a tube, pipe, circular or a flat plate for collecting the charged fine particulate; (iii) a dust removal device (dry type) or a spray device (wet type) for separating collected fine particulate from the collector electrodes; (iv) a hopper or a trough for collecting the separated fine particulate; and (v) a power source for supplying the power to the electrostatic precipitator to cause the required electric collection fields and corona discharges. A dust removal device is normally used with a dry electrostatic precipitator where collector electrodes are rapped by a hammer-like device to dislodge collected fine particulate. The discharged particulate is then stored into a collection unit such as a hopper or a trough provided in a lower portion of the device.
In the wet type device, fine particulate collected onto the collector electrodes is washed and removed by an injected cleaning solution such as water. When a large amount of the fine particulate has collected onto the collector electrodes and not removed, the Coulomb force for attracting the charged fine particulate may be reduced thereby reducing the collection efficiency. In addition, if the weight of the dust accumulated on the collector electrodes exceeds the electrical (Coulomb) forces holding dust on the collector electrodes, a random dust dislodging may take place resulting in the dust re-entrainment, increased emissions, etc. Therefore, in order to prevent the case where the dust collection cannot be performed in a stable state, the dry and wet types of removal are normally used to remove the fine particulate from the collector electrodes.
In recent years, various apparatus types in which discharge, collector electrodes and filters or mechanical collectors are housed in a common housing have been used. In this type of “hybrid” collector, the electrostatic precipitator and mechanical filter both work synergistically to assist each other in a common goal to reach ultra-fine particulate collection efficiency. Consequently, the overall emissions of fine particulate are significantly reduced from those of non-hybrid types of collectors.
The collectors that have been described generally remove particular matter from the exhaust gas flow. In addition, there are processes intended to also remove polluting gases from the flow. One example is the wet desulfurization process in which a flue gas is contacted with a solution or slurry containing an absorbent for removing air pollutants such as SO2 and fly ash. Various such processes have been proposed, and a number of large commercial apparatuses are currently deployed for the treatment of flue gas from thermal power boilers, industrial and other commercial operations. Processes in which limestone is used as the absorbent and in which gypsum is produced as a by-product are most commonly used.
Specifically, the process known as CT-121 in which a flue gas is efficiently purified by totally treating not only SO2 but also other air pollutants which include fly ash, HCl and HF and which are contained in a large amount in coal-fired boiler flue gas includes the following steps (See U.S. Pat. No. 4,911,901):                i. introducing the flue gas into the scrubber vessel through a vertical, open-ended pipe to form an annular jet stream or a gas-continuous flow accompanied with liquids and solids, so that portions of the SO2 and the fly ash contained in the flue gas are transferred to the liquid;        ii. sparging the annular jet stream from the vertical, open-ended pipe into a pool of aqueous absorbent held in a well-mixed vessel to form a jet bubbling layer or a liquid/solid/gas three phase mixed layer, which contains fine bubbles in the liquid-continuous phase of a shallow upper zone of the aqueous absorbent, so that a greater part of the SO2 and the fly ash are removed from the gas;        iii. dispersing air into the pool in the reactor below the jet bubbling layer to provide streams of fine oxygen-containing gas bubbles throughout the whole vessel including the jet bubbling layer, so that the absorbed SO2 and other sulfites are oxidized to form coarse gypsum crystals while chemical oxygen demand of the absorbent is reduced; and        iv. withdrawing a portion of the aqueous absorbent to maintain the concentration of the gypsum in a predetermined range.        
The exhaust gas leaving the jet bubbling layer enters the final cleaning stage and after removal of entrainments in a mist eliminator; the purified gas is then discharged to the atmosphere. However, the above system may also require additional steps of the post mist-elimination and fine particulate collection.
Should the mechanical, scrubbing and electrostatic collectors be connected in series to achieve the desired total results, an elaborate ductwork becomes necessary to allow exhaust gas to flow through the mechanical filter, the scrubber and then through the electrostatic precipitator or vice versa. Such arrangements are very costly and cumbersome, and they are inherently less efficient especially in a sub-micron and nano-particulate size range.
It would be advantageous to have a hybrid collector in a compact shape that could provide an integrated system for minimizing pollution which synergistically combines a mechanical filter such as a barrier filter with a two-stage electrostatic precipitator, and wherein the mechanical cleansing action of the barrier filter is rendered compatible with that of the precipitator charging and removing fine particles and with that of a flowing gaseous stream through a pool of liquid to scrub it of the gaseous pollutants. It would also be advantageous to have a hybrid collector where the precipitator is aerodynamically reconciled, and the resultant system attains optimum efficiency and functions synergistically as a single unit to remove a full spectrum of contaminants from the gas stream.